The in vitro antibacterial activity of the new fluoroquinolone Bay y3118 against 609 clinical isolates was evaluated. Bay y3118 exhibited activity against a broad spectrum of organisms, including Gram-negative bacilli, Gram-positive cocci, mycobacteria. The activity of Bay y3118 was often superior to that of other quinolones. Against Gram-negative bacilli its activity was similar to that of ceftriaxone, cefotaxime, ceftazidime and imipenem except for Serratia marcescens, Klebsiella pneumoniae, Enterobacter spp. and Xanthomonas maltophilia, where its activity was superior. Gentamicin and piperacillin sometimes were less active. Bay y3118 was active against a large number of Gram-positive cocci. The fluoroquinolones tested were active against all the strains of Mycobacterium tuberculosis, but only Bay y3118 was effective against Mycobacterium avium.